1. Field of the Invention
This invention relates to a method and apparatus for observing an element distribution and more particularly to a method and apparatus for observing an element distribution through computation under utilization of some core loss electrons.
2. Description of the Related Art
An incident electron beam sometimes loses energy under an interaction between the electron beam and a specimen. As this phenomenon, there are various kinds of methods such as a plasmon loss, a core loss and braking radiation, and their lost energy is different in reference to a structure of the specimen or the type of composing element and the like. In these methods, the core loss electron in particular showed a predetermined energy loss value in response to the type of element or their connected state and thus this core loss electron has been applied for observation of the element distribution or analysis of the connected state.
As an apparatus for analyzing an element under utilization of its energy loss, there have been provided an electron energy-loss spectroscopy (EELS) device (R. F. Egerton: Electron Energy-loss Spectroscopy in the Electron Microscope, Plenum Press (1986)) or an energy-filtering transmission electron microscopy (L. Reimer ed.: Energy-Filtering Transmission Electron Microscopy, Springer (1995)).
EELS device has a spectrometer for dividing energy of element in a spectroscopic manner mounted at a rear part of an observing apparatus utilizing some transmission electrons, such as a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM). As a well-known structure of the spectrometer, it has been disclosed in U.S. Pat. No. 4,743,756 and of Japanese Patent Laid-Open No. Hei 7-21,966.
As the energy-filtering transmission electron microscope, there have been provided an in-column type in which a focusing-type energy filter is inserted at the midway part of a focusing system of TEM, and a post-column type in which a focusing energy filter device is arranged at a rear part of the focusing system in TEM. As the in-column type, there have been provided Ω-type (Japanese Patent Laid-Open No. Sho 62-66,553), kα-type (Japanese Patent Laid-Open No. Sho 62-69,456), kγ-type (Laid-Open Patent Application WO 96/02935) and a mandolin-type (Japanese Patent Laid-Open No. Hei 7-37,536) or the like. Although as the post-column type, an apparatus called an imaging filter manufactured by GATAN Co., Ltd. (Gatan Imaging Filter: GIF (O. L. Krivanek. A. J. Gubbens and N. Dellby: Microsc. Microanal. Micostruct. vol. 2(1991)315.)) is the most famous device, there is also provided a system for drawing Ω-type orbit (Japanese Patent Laid-Open No. Hei 11-073899 and Japanese Patent Laid-Open No. 2001-243910).
A principle will be described at first in which an element distribution of specimen is attained under application of energy-loss electrons. At first, in FIG. 4 is indicated a typical energy-loss spectrum near the core loss energy. In the case that energy is lost during a method not related to an element such as a braking radiation or the like, a background having no specific energy loss value, but having a continuous spectrum is formed. This intensity J is approximately defined by the equation (1).J=A−exp(−r·E)  (1) where, E is an energy loss value, and A and r are constants defined by a thickness of the specimen and its composition. Although an intensity of core-loss electrons is overlapped on this background, energy lost while exciting inner core electrons of the specimen is more than a minimum requisite energy for excitation, so that a shoulder called a core loss edge is normally formed in the energy loss spectrum. An image attained by an electron beam ranging from an energy loss area E1 to E1+ΔE is defined as a pre-pre-edge image, an image attained by an electron beam ranging from an energy loss area E2 to E2+ΔE is defined as a pre-edge image and an image attained by an electron beam ranging from an energy loss area E3 to E3+ΔE is defined as a post-edge image. Since the pre-pre-edge image does not contain any core loss electrons, it is composed of only the background. When an intensity of the pre-pre-edge image is defined as I1, a following equation of                                                                                                    I                  1                                =                                ⁢                                                                            ∫                                              E                        1                                                                                              E                          1                                                +                                                  Δ                          ⁢                                                                                                           ⁢                          E                                                                                      ⁢                    JdE                                    =                                                            ∫                                              E                        1                                                                                              E                          1                                                +                                                  Δ                          ⁢                                                                                                           ⁢                          E                                                                                      ⁢                                                                  A                        ·                                                  exp                          ⁡                                                      (                                                                                          -                                r                                                            ·                              E                                                        )                                                                                              ⁢                                                                                           ⁢                                              ⅆ                        E                                                                                                                                                                    =                                ⁢                                                      A                    r                                    ⁢                                      exp                    ⁡                                          (                                              -                                                  rE                          1                                                                    )                                                        ⁢                                      {                                          1                      -                                              exp                        ⁡                                                  (                                                                                    -                              r                                                        ⁢                                                                                                                   ⁢                            Δ                            ⁢                                                                                                                   ⁢                            E                                                    )                                                                                      ]                                                                                                          (        2        )            Similarly, intensity I2 of the pre-pre image becomes                                                                         I                2                            =                            ⁢                                                A                  r                                ⁢                                  exp                  ⁡                                      (                                          -                                              rE                        1                                                              )                                                  ⁢                                  exp                  ⁡                                      (                                                                  -                        r                                            ⁢                                                                                           ⁢                      δ                      ⁢                                                                                           ⁢                                              E                        1                                                              )                                                  ⁢                                  {                                      1                    -                                          exp                      ⁡                                              (                                                                              -                            r                                                    ⁢                                                                                                           ⁢                          Δ                          ⁢                                                                                                           ⁢                          E                                                )                                                                              ]                                                                                                        =                            ⁢                                                exp                  ⁡                                      (                                                                  -                        r                                            ⁢                                                                                           ⁢                      δ                      ⁢                                                                                           ⁢                                              E                        1                                                              )                                                  ⁢                                  I                  1                                                                                        (        3        )            When a core loss electron intensity contained in the post-edge image is defined as Ie and a background intensity is defined as Ibk, an intensity I3 of the post-edge image is obtained by the following equation:                                                                         I                3                            =                            ⁢                                                I                  e                                +                                  I                  bk                                                                                                        =                            ⁢                                                I                  e                                +                                                      exp                    ⁡                                          (                                                                        -                          r                                                ⁢                                                                                                   ⁢                        δ                        ⁢                                                                                                   ⁢                                                  E                          2                                                                    )                                                        ⁢                                      I                    2                                                                                                          (        4        )            When a contrast dependent on Ie of the equation (4) can be calculated, an element distribution image can be attained.
(a) 3-Window Method:
At first, a ratio between the equation (3) and the equation (2), i.e. R1 is defined as R1=I2/I1=exp(−rδE1), r becomes                     r        =                              1            ⁢                          nR              1                                            δ            ⁢                                                   ⁢                          E              1                                                          (        5        )            so that Ie can be defined as                               I          e                =                              I            3                    -                                    R              1                                                δ                  ⁢                                                                           ⁢                                      E                    2                                                                    δ                  ⁢                                                                           ⁢                                      E                    1                                                                        ⁢                          I              2                                                          (        6        )            In particular, when an equation of δE1=δE2 is defined, the equation (6) can be simplified as follows.                               I          e                =                              I            3                    -                                    I              2              2                                      I              1                                                          (        7        )            This is a method called 3-window method and this is most widely used.
(b) Ratio Map Method
A ratio R2 between the equation (4) and the equation (3), i.e. R2=I3/I2                              R          2                =                                            I              e                                      I              2                                +                      exp            ⁡                          (                                                -                  r                                ⁢                                                                   ⁢                δ                ⁢                                                                   ⁢                                  E                  2                                            )                                                          (        8        )            may also provide a measure in regard to an element. Because Ie=0 can be attained in an area having no element, so that only exp (−rδE2) is attained and in turn, information on Ie is overlaid in an area having an element. When r is constant in an observation area, only information Ie becomes a contrast and is observed. This method is called either a Ration map or 2-window method.
(c) Spectrum Map Method
Although the aforesaid two types have been indicated on the basis of a total number of electrons in a certain energy area, if, the spectrum itself could be recorded, the background area is fitted by the equation (1) and a value Ibk can be calculated more precisely. A method for recording all the spectra for every one point on the specimen under application of a scanning function of STEM and calculating Ie for each of the points is called a spectrum mapping method.
(d) Imaging EELS Method
In the energy filter TEM, an energy loss area is determined under application of the energy selection slit. It is satisfactory to take a photograph of two images for the ratio mapping method and to take a photograph of three images for the 3-window method. However, it is also possible to take many photographs with a narrower slit and to attain an element distribution image under a procedure of the spectrum mapping method. This method is called an imaging ELS method.
In the gazette of Japanese Patent Laid-Open No. 2001-148231 is disclosed a structure for use in simultaneously detecting electron intensities in a plurality of energy loss areas. Upon detection of the electron intensities, it is possible to attain an element distribution image in concurrent with the STEM by calculating it not through software, but by an electric circuit.
Some features in the four types of element distribution image calculating methods described in the aforesaid prior art are indicated in Table 1. An independent detection in the Table is meant by a system for separately detecting an intensity of electron beam in a different energy loss area, and a simultaneous detection is meant by a system for detecting it under an application of a plurality of detection elements as described in the gazette of Japanese Patent Laid-Open No. 2001-148231. QE is meant by “quite excellent”; E is meant by “excellent”; and I is meant by “inferior”, respectively. Artifact is meant by a false contrast generated in the case that r in the equation (8) is not kept constant, this artifact appears due to a mere difference in density even if the specimens have the same thickness to each other and this artifact applies a substantial influence against a quantitative characteristic. A diffraction contrast is meant by a phenomenon in which the number of electrons varies because the electron beams diffracted by the specimen are removed by an object iris diaphragm to influence against its quantitative characteristic. In the case of ratio map method, no influence is applied to a result even if an absolute amount changes because a ratio of the number of electrons is computed. However, in the case of other methods, it is necessary to pay an attention so as not to have any diffraction contrast at a stage of attaining an image before calculation because a subtraction method is to be carried out.
TABLE 1                  Features of Method for Calculating Various Types of Element DistributionsMethod3-window methodRatio map methodSpectrumIndependentSimultaneousIndependentSimultaneousmapImagingItemdetectiondetectiondetectiondetectionmethodEELSQuantitativeEEIIQEEcharacteristicArtifactEEIIQEEDiffractionIIEEIIcontrastPrecisenessEEEEIESensitivityEEEBQEES/NIIQEQEEESpeedEQEEQEIIPositionalEQEEQEIIalignmentApparatusEQEEQEIIstabilityQE: Quite excellent, E: Excellent, I: Inferior 
As apparent from this table, the method of the present invention has no superior characteristic in all items. The present invention becomes a powerful tool only through a simultaneous accomplishment of at least one merit in which the ratio map method is not influenced by a diffraction contrast and the other merit in which no artifact is present in other systems.